A helmet of the type used by firemen and the police must often be used together with a respirator mask, that is with a face mask provided with appropriate air filters or supplies and having a clear lens in front of the eyes.
Typically the helmet is provided with an interior web assembly that supports its hard outer shell wholly out of contact with the user's head. The respirator has a flexible skirt that must engage snugly against the face of the user and is normally held in place by its own straps for an airtight fit.
Removing the helmet, putting on the respirator, and then donning the helmet again is normally considered inacceptable, particularly in the dangerous environment in which such an assembly is used, and since the helmet does not fit properly over the respirator straps. Specially configuring the mask so it can fit within the face hole of the helmet has been suggested, but no known system allows a respirator mask to fit safely with the whole range of head sizes that the helmet can be used with, and the procedure for attaching and removing the respirator is cumbersome.